One area where the use of plastic containers has become widespread is in the food packaging industry. Accordingly, it is common for these plastic food containers to serve as the end display package in which the product is presented for sale to the customer. Typical of these containers are those used for dairy products such as cottage cheese, sour cream, or the like where an integral body of the container is provided having a sidewall that tapers down to an integral transverse bottom wall with the top opening being closed by a plug fit lid. Normally, the lid has a depending peripheral skirt which locks onto the upper rim of the tapered wall of the container body. One difficulty encountered in the design of closure lids for the containers described above is ensuring that when they are nested in a stack such as for shipping, the stacked lids maintain a constant gap between adjacent skirts of respective stacked lids. Maintaining uniform gap spacing is important to allow them to be efficiently utilized with high-speed automated packaging equipment which position the containers for automatic filling and automatically caps the filled containers with lids taken from the stack by mechanical devices of the equipment. Various rib structures have been employed with lids to provide the requisite spacing, see e.g. U.S. Pat. Nos. 4,826,039 and 5,377,861.
Many of these food product plastic containers have their parts formed by a thermoforming process. In thermoforming, a thin plastic sheet is formed into the desired shape by heating and forcing the sheet against a mold to produce a container part having a uniform, very thin, cross-sectional thickness which can result in a part having very flexible walls. In the particular application of interest herein, currently there is a flavored yogurt thermoformed container that has a reverse tapered sidewall with a larger diameter bottom and being open at both the smaller diameter top and at the bottom thereof. A separate bottom closure member is also formed by thermoforming and is spinwelded to the sidewalls to close off the bottom for receiving yogurt therein. The bottom closure includes a base panel and depending skirt wall which is spinwelded to the interior surface of the body wall to permanently attach the pieces together. Thus, unlike the previously described top closure lids which are designed to be opened, the bottom closure for this particular yogurt container does not have a locking skirt which locks onto a rim of the sidewall and which can be opened to gain access to the food therein. Instead, after being filled with yogurt, the top is closed by a flexible foil seal as by an adhesive. To gain access to the yogurt, the seal is peeled open from over the opening at the top of the container sidewall.
The thermoformed and spinwelded yogurt containers described above suffer from numerous shortcomings. For spinwelding the bottom closure to the container body wall, both pieces are provided with integral gripping lugs which project relatively far radially inward relative to the body and skirt walls so that they can be grasped by the spinwelding equipment for rotating the two parts relative to each other to create frictional heating for welding the parts together. The bottom closure has the lugs formed on its skirt, and the body sidewall has lugs around the top thereof. The spinwelding technique requires specialized handling and filling equipment that results in a relatively slow production of containers for filling.
Since the parts of the above-described yogurt container are thermoformed parts having a constant wall thickness, the radially inward projecting lugs form corresponding indentations on their exterior wall surfaces. Because of the aforementioned display function of the exterior surface of the yogurt container, maximizing the amount of surface area available for printing information, such as product characteristics, e.g. ingredients, nutritional content, or other required information about the product, is an important consideration, especially where the containers are relatively small, such as for example with the preferred 6 oz. (170 g) yogurt containers herein. The lugs at the top of the wall restrict the height of the printing that can be received on the container sidewall. In addition, there are unsightly indentations on the sidewall due to the lugs that are readily visible to the purchaser, and because of the radial extent to which they project into the container interior, they can unduly interfere with removing the food product therefrom, e.g. spooning yogurt out from the container. Accordingly, there is a need for a plastic container for food products such as a yogurt container which is more aesthetically pleasing, and better maximizes the print receiving surface area thereon.
Another significant characteristic the containers should possess is the ability to stack with uniform spacing between the container parts and so that while stacked, adjacent parts do not become jammed and wedged together. In addition, the space taken up by a given number of stacked container parts should be minimized. The above described lugs of the prior yogurt container, in addition to their grasping function for spinwelding provide a stacking surface with their bottom flat surfaces. The indentations of the lugs extend relatively far down the sidewalls spaced from the top in both the container body and the bottom closure. This precludes stacking of these container parts in a compact fashion.
It is also important that the container be adapted to be used with conventional automated container feeding equipment that is currently employed in container filling assembly and printing operations. Both the upper container wall and the bottom closure lack an annular rim for feeding with mechanical mechanisms or devices such as mechanical fingers, feed screws or shuttles of the automated feeding equipment commonly used with the packaging of dairy products.
As previously mentioned, thermoformed plastic containers generally have very thin cross-sectional thickness so that their walls can be very flexible. This is especially true with the bottom closure of the above described yogurt container where the skirt sidewall is relatively thin in thickness and the base panel is quite wide in diameter which can cause the closure member to be very pliable.